1. Field of the Invention
This invention relates to a process and machine for effecting solder reflow operations and, in particular, to a process and machine for reflowing solder plated continuous flexible circuit webs with negligible loss of heat transfer fluid.
2. Description of the Prior Art
Several methods have been disclosed in the prior art for effecting solder reflow operations on printed circuits through the use of hot saturated vapors. One such method is briefly described in an article entitled "Solvent Vapor Solder Reflow" by E. G. Dingman, appearing in the IBM Technical Disclosure Bulletin, Vol. 13, No. 3, August 1970, at page 639. Dingman discloses use of boiling solvents to rapidly and selectively apply heat to small areas having high thermal conductivity to enable solder rework operations with materials and components that are heat sensitive. It seems readily apparent that Dingman does not address the problems of handling large and continuous flexible circuit webs or loss of the boiling solvent.
One method for continuously handling printed circuits is disclosed in R. C. Pfahl, Jr. et al U.S. Pat. No. 3,866,307, issued Feb. 18, 1975. In this method individual circuit boards are loaded onto a conveyor and passed through a receptacle containing hot saturated vapors of an expensive fluid and a wave soldering font. Individual circuit boards are heated by the vapors and skim the solder wave at a low point of the conveyor catenary. One problem resulting from the Pfahl, Jr. et al approach is that solder tends to pool at the low point of the catenary. Another problem is that despite attempts to retain the expensive fluid, substantial quantities are dragged out of the receptacle along with the conveyor and the circuit boards themselves.
Another method disclosed in T. Y. Chu et al U.S. Pat. No. 3,904,102, issued Sept. 9, 1975, attempts to reduce loss of the expensive fluid by use of a less expensive vapor blanket atop the primary vapor zone. One embodiment of the Chu et al method utilizes batch processing techniques. A group of printed circuits is lowered into a receptacle containing the primary vapor zone and the secondary vapor blanket. In another embodiment a conveyor carries the individual circuits into the vapor zone. However, in both embodiments significant quantities of the expensive primary fluid are still lost. Moreover, the second embodiment continues to suffer from solder pooling effects. The first embodiment obviously is not readily adaptable for handling continuous webs of printed circuits.
A somewhat related application of the use of hot vapors is disclosed in K. W. Kamena U.S. Pat. No. 3,737,499, issued June 5, 1973. The Kamena method is used for modifying plastic surfaces on articles of manufacture. An individual plastic article is inserted into a multicompartmented chamber containing one or more vapor regions. The heated vapors impinge on the surfaces of the plastic articles and dissolve at least a molecular layer to remove any surface blemishes and produce a smooth, continuous finish. Kamena, like Pfahl, Jr. et al and Chu et al, also suffers loss of the vapor material through web dragout.
Accordingly, it is one object of the present invention to implement solder reflow operations on a continuous, flexible circuit web without solder pooling.
Another object is to virtually eliminate any distortion of the web dimensions during solder reflow operations.
Still another object of the present invention is to substantially reduce the possibility of dielectric deterioration caused by the solder reflow process.
Yet another object is to virtually eliminate solder slivers produced during etching operations.
A further object of the present invention is to reveal any discontinuities in the printed circuit which may have been bridged by solder during the solder plating process.
Still a further object of the present invention is to indicate the solderability of the flexible circuits.
Yet another object is to facilitate visual inspection of the flexible circuits.
Still a further object is to improve the appearance of the flexible circuits.
An even further object is to significantly reduce, if not virtually eliminate, any loss of the expensive working fluid resulting from web dragout, diffusion and convection.